1. Field of the Invention
The present invention relates to a binder-loaded type charge-transport liquid crystal material which comprises a liquid crystal compound and a binder and, in spite of the presence of the binder, can retain the properties inherent in the liquid crystal compound, and devices or elements using the same.
2. Background Art
Liquid crystal compounds have various useful properties in addition to an inherent ability to form a liquid crystal, and thus are expected to be utilized in a variety of devices or elements. One of these useful properties is a high level of charge-transport properties to which attention has recently been drawn for utilization as organic semiconductors. Most properties of the liquid crystal compound, however, are deteriorated upon the inclusion of impurities in an amount on the order of ppm. For this reason, combining the liquid crystal compound with other material(s) to prepare a polyfunctional liquid crystal-containing composition has hardly been carried out. For example, an attempt to bring a liquid crystalline charge-transport material to a solid form has resulted in failure due to a rapid deterioration in carrier mobility upon the incorporation of an impurity, such as a binder, into the liquid crystal compound. For this reason, the liquid crystal compound as such should have been sealed into a cell.
Accordingly, the present invention has been made with a view to solving the problem of the prior art, and it is an object of the present invention to provide a binder-loaded type charge-transport liquid crystal material which comprises a liquid crystal compound and a binder and, in spite of the presence of the binder, can retain the properties inherent in the liquid crystal compound, and devices or elements using the same.
The present inventors have found that a combination of a liquid crystal compound with a specific binder can realize the retention of the properties inherent in the liquid crystal compound in the charge-transport liquid crystal material and, at the same time, can develop the effect of the binder, which has led to the completion of the present invention.
Thus, according to one aspect of the present invention, there is provided a binder-loaded type charge-transport liquid crystal material comprising a liquid crystal compound and a binder, said binder-loaded type charge-transport liquid crystal material substantially retaining properties inherent in the liquid crystal compound.
Liquid crystal compounds usable in the present invention include, for example, those having charge-transport properties which are expected to be applied to various devices or elements.
Liquid crystal compounds having such charge-transport properties include, but are not particularly limited to, for example, aromatic ring-containing liquid crystal compounds. Specific examples thereof include 2-(4xe2x80x2-pentyloxyphenyl)-6-decyl-oxynaphthalene, 2-(4xe2x80x2-pentyloxyphenyl)-6-octyl-oxynaphthalene, 2-(4xe2x80x2-pentyloxyphenyl)-6-hexyl-oxynaphthalene, and 2-(4xe2x80x2-octyloxyphenyl)-6-dodecyl-oxynaphthalene. The liquid crystal compound according to the present invention is typically one having a smectic phase, but is not limited to this only.
For example, carrier mobility will be discussed among the properties of the liquid crystal compound. The liquid crystalline molecule has a self-aligning property by virtue of the molecular structure. Therefore, unlike the molecule dispersed material, the use thereof as a hopping site inhibits spatial and energetic dispersion of the hopping site and can realize band-like transport properties such as found in molecular crystals. This can offer a feature that larger mobility than that in the conventional molecule dispersed materials can be realized and, in addition, the mobility does not depend upon the electric field.
Liquid crystal compounds, which are preferred in terms of carrier mobility, include those having an electron mobility of not less than 1xc3x9710xe2x88x925 cm2/Vs and those having a hole mobility of not less than 1xc3x9710xe2x88x925 cm2/Vs. Various devices produced using liquid crystal compounds having such mobility are expected to have quick response.
According to the present invention, the binder refers to a material, which when incorporated into a liquid crystal compound, can vary, for example, the viscosity, fluidity, or elasticity of the liquid crystal compound. Any binder may be used in the present invention without particular limitation so far as the binder does not substantially vary specific properties of the liquid crystal compound.
Such binders include, for example, various organic polymers. Specific examples thereof include those having an aromatic ring and/or a condensed aromatic ring and/or a heterocyclic aromatic ring on the main chain or side chain thereof. Preferred examples of such binders include those comprising a material(s) selected from the group consisting of polystyrene and derivatives thereof, polynaphthalene and derivatives thereof, polyester resin and derivatives thereof, alkyd resin and derivatives thereof, polycarbonate resin and derivatives thereof, phenolic resin and derivatives thereof, xylene resin and derivatives thereof, epoxy resin and derivatives thereof, urethane resin and derivatives thereof, polyvinylcarbazole and derivatives thereof, polyvinylpyridine and derivatives thereof, amino resin and derivatives thereof, polythiophene and derivatives thereof, urea resin and derivatives thereof, copolymers produced from two or more of monomers constituting said resins, and mixtures of two or more of said materials. More specific examples thereof include polystyrene.
The content of the binder in the binder-loaded type charge-transport liquid crystal material is not particularly limited, and the liquid crystal compound and the binder may be mixed together in any ratio, so far as the properties inherent in the liquid crystal compound are substantially retained. In this case, the amount of the binder is not limited to such an amount that the binder enables the binder-loaded type charge-transfer liquid crystal material to be brought to a solid form or to form a coating, and the amount of the binder may be too small to improve the viscosity. Preferably, however, for example, the use of the binder in an amount of not less than 20% by weight is suitable because the binder-loaded type charge-transfer liquid crystal material can be coated to form a film while retaining the properties of the liquid crystal compound.
The binder-loaded type charge-transfer liquid crystal material according to the present invention is preferably such that the liquid crystal compound and the binder can be mutually dissolved (the term xe2x80x9cmutually dissolvedxe2x80x9d as used herein means that the two materials are in the state of dissolution in each other to such an extent that phase separation is not observed at least microscopically). In this connection, it should be noted that the liquid crystal compound and the binder, which are mutually soluble (compatible), are not always required to be mixed together to such an extent that they are mutually dissolved. In short, according to this preferred embodiment, the liquid crystal compound and the binder constituting the charge-transport liquid crystal material are soluble in each other, independently of whether or not the liquid crystal compound and the binder after mixing are in the mutually dissolved state. According to this preferred embodiment, the high level of carrier mobility of the liquid crystal compound can be retained after the incorporation of the binder into the liquid crystal compound.
Among the properties of the binder-loaded type charge-transport liquid crystal material, the carrier mobility is basically the same as that described above in connection with the liquid crystal compound, and is preferably such that the electron mobility is 1xc3x9710xe2x88x925 to 1 cm2/Vs and the hole mobility is 1xc3x9710xe2x88x925 to 1 cm2/Vs. In the present specification, xe2x80x9csubstantially retainingxe2x80x9d the properties according to the present invention may also be expressed, for example, such that the liquid crystal compound and the binder-loaded type charge-transfer liquid crystal material meet the same requirement(s) or can be used in the same application using the properties.
The binder-loaded type charge-transfer liquid crystal material according to the present invention may have fluidity. Preferably, however, for example, the binder is added in a large amount to bring the binder-loaded type charge-transfer liquid crystal material to a solid state. The solid is preferably in a coating form.
The binder-loaded type charge-transfer liquid crystal material according to the present invention may be used in various applications without particular limitation, for example, in driving paths of various devices. Such devices include, for example, image display devices, EL (electroluminescence) devices, photoconductors, space modulating devices, thin-film transistors, and photosensors.